Obviously, clipping points will be different, but unclipped, rendered tones will be different, too, which proves beyond a doubt that there is no simple linear adjustment to the data with the "exposure" slider. ACR does not allow you to expose all the way to the right, and get a reasonable render with negative exposure compensation. You need another converter for that.
Like everything Adobe, ACR has some great features, and some big, gaping holes.
Well, I did some experiments with overexposures of a MacBeth Color Checker exposed under daylight at 2, 3, 4, and 8 seconds with a Nikon D200. The 2 second exposure gave proper results with ACR at default settings. The white patch RGB ProPhotoRGB reading of the 2 s exposre was 247, 248, 246. I converted the NEFs with ACR into ProPhotoRGB with the exposure adjusted to give a reading of 248 from the remaining white patches, except for the 8 second exposure (+2 EV) where I used an exposure adjustment of -2. With this degree of overexposure, it is not possible to bring the white square below 255.
I also converted the NEFs with Iris 5.33 and recorded the 12 bit raw values.
The results are shown in the table below. W1 is the white patch on the left and W2 is the next patch to the right. McR, McG, and McB are the red, green, and blue patches respectively. The RGB values and saturation (from HSB readout of PSCS2) are shown in the table below. The image is in sRGB for web display (converted from the ProPhotoRGB)
With +0.6 EV overexposure, W1 begins to blow in the green channel, and with 1 EV of overexposure, the green channel is blown in both W1 and W2 as shown by the raw readouts. However, even at +2EV none of the RGB patches were blown in any of the RGB channels and the color values and saturation of all these patches were virtually the same up to - 1 EV of exposure compensation, and changed only slightly even with -2 EV.
Therefore, I conclude that exposure compensation in ACR is linear in real world shooting situations with up to one stop of highlight recovery. Above this, the conversion is nonlinear, since one can not bring the white patch down under 255. According to Bruce Fraser in his ACR with PSCS2
, one should expect no more than 0.5 to 1 stop of recovery with ACR. I conclude that ACR highlight recovery produces good results with ETTR under these conditions, and one can shoot fully to the right within these limits.